The present invention relates to a heavy loading tubeless tire and, more particularly, to a heavy loading tubeless radial tire having a bead structure which allows less deformation even after a long use.
In the prior art, the heavy loading tubeless tire to be used in trucks or buses is constructed, as shown in FIG. 3, of a bead portion including: a carcass ply 2 wound around a bead core 1 from the inside to the outside of the tire; a reinforcing layer 3 arranged along the carcass ply 2; a reinforcing layer 6 covering the terminals of the carcass ply 2 and the reinforcing layer 3; bead fillers 4 and 5 filling up the upper portion of the bead core 1; and a rubber layer enveloping the foregoing components altogether. Generally speaking, the base 9 of the bead portion is composed of a region W.sub.1, which is located at the side of a bead heel 8 and inclined at such an angle .theta..sub.1 of inclination with respect to the axis of tire rotations as is substantially equal to that of the (not-shown) rim base of the tire rim, and a region W.sub.2 which is located at the side of a bead toe 7 and inclined at such an angle .theta..sub.2 as is larger by 3 to 15 degrees than that of the rim base. In order to hold the gas-tightness, the heavy loading tubeless tire is fitted for use on the rim having its base inclined usually at about 15 degrees with respect to the axis of tire rotations and has the bead portion desired to have an internal diameter smaller than the diameter of the rim. In order to facilitate the inflation, when the rim is to be assembled, and to enhance the gas-tightness, the heavy loading tubeless tire is formed with the aforementioned region W.sub.2 which is inclined at an angle larger by 3 to 15 degrees than that of the bead base 9.
In the case of the heavy loading tubeless tire, i.e., the radial tire in which the carcass chords are arranged substantially in the normal direction of the annular bead core, the force to be exerted upon the carcass chords while the tire is being inflated or rolling under load is transmitted directly to the bead core so that a remarkably strong force is applied in the normal direction of the bead core. In case the tire of the prior art having the aforementioned bead structure is fitted on the rim, the tire components positioned below the bead core 1 are compressed by the bead structure so that the force to the compression is exerted upon the bead core 1. As shown in FIG. 6, however, the force distributed in the widthwise direction of the bead core 1 (i.e., in the direction of tire rotations) is not uniform, as indicated by arrows, such that the force is stronger in the region W.sub.2 than in the region W.sub.1. As shown in FIG. 7, moreover, the force, which are generated in the direction of arrows by the tension (as indicated by broken arrows) acting upon the carcass chords 2, are exerted upon the bead core 1 to float the toe of the bead core 1 together with the aforementioned force to the compression.
Since this fact is not known yet, the heavy loading tubeless tire of the prior art is constructed by laminating a plurality of bead wires 11 to make a hexagonal bead core 1 so that the widthwise strength of the bead core 1 may not be different between the sides of the bead toe 7 and the bead heel 8. However, this tire is short of strength at the side of the bead toe 7 of the bead core 1 so that the bead core 1 itself cannot be prevented from being bent and deformed radially outward at the side of the bead toe 7. In other words, the bead core 1 having a section as indicated by the solid lines in FIG. 8 tends to be deformed to have a section as indicated by broken lines. Moreover, the rubber at the bead toe is repeatedly compressed and deformed by the bending deformations of the bead core rolling under load and is degraded as a result of the temperature rises of the internal air and the rim of the tire in accordance with the run of the vehicle. Thus, the deformations are frequently so permanent that they cannot be remedied. Since, moreover, the strength of the bead toe 7 of the bead core is not sufficient, as has been described hereinbefore, the portion of the bead core at the side of the bead to 7 is deformed by the force coming from the rim and exerted upon the bead core and the carcass tension. This deformation reduces the area of the bead base 9 to come into contact with the rim base 9 thereby augment the contact pressure of the bead base 9 per unit width. As a result, at the initial stage, the deformation of the bead portion is caused at a remarkably narrow region at the leading end of the bead toe, as shown in FIG. 9. This deformation progresses from the shape of FIG. 9 through the shape of FIG. 10 to the shape of FIG. 11 and extends from the bead toe to the bead heel. Moreover, the widthwise deformation T and the radial deformation S of the bead toe are gradually augmented. As shown in FIG. 11, a floating deformation of a distance u is also caused at the bead toe of the bead core 1. Incidentally, broken lines of FIGS. 9 to 11 designate the bead toe of a new tire.
As a result of the aforementioned deformations of the bead core and toe, the area of the bead base to contact with the rim base is reduced to float the bead toe thereby to drop the gas-tightness of the tire, and the tire becomes difficult to inflate when the rim is assembled Moreover, the region W.sub.2 of the new tire having a higher gradient than the rim base to increase the diameter of the leading end of the bead base substantially. On the other hand, the deformations of this kind are caused neither simultaneously nor equally along the bead base in the circumferential direction of the tire so that the bead base is not evenly fitted on the rim base, thus raising a major cause for generating oscillations during the run of the vehicle.
If, moreover, the contact area of the bead base and the rim base is reduced by the bending deformations of the bead core, the carcass chords fixed firmly below the bead core become loose to increase the motions of the turned-up terminal portion of the carcass layer. Thus, there arises a serious defect that the turned-up terminal portion of the carcass layer are liable to separate.
These problems caused by the structures of the bead portion and the bead core are serious ones to be solved in the heavy loading tubeless tire which is renewed for use with a view to elongating the lifetime.
Japanese patent application Kokai publication No. 61-295107 has proposed a tire in which the bead wires in the bead core are made to have different diameters in the radially inner and outer sides so as to equalize the stresses of the individual layers in the bead core. However, this proposal has neither noted the fact that the aforementioned stress distribution in the bead core is remarkably uneven not in the vertical direction but in the widthwise direction nor solved the aforementioned problems.
On the other hand, Japanese patent publication No. 62-9442 has proposed a tire which has its bead toe prevented from being deformed by regulating the positional relations at the bead base of the bead core having a hexagonal section. However, this proposal has not noted the shape of the bead base and the strength of the region W.sub.2, which are fundamental causes for deforming the bead toe of the heavy loading tubeless tire.
There is known a tire which has a bead core 1 prepared by bundling a plurality of bead wires 11 having square sections in a generally rectangular form, as shown in FIG. 5. This tire has a slightly higher strength at the side of the bead toe of the bead core than the aforementioned tire having the bead core of generally hexagonal sections, but the strength per se is not sufficient for preventing the deformation of the bead toe so that the troubles similar to those of the aforementioned heavy loading tubeless tire are caused if the use period of the tire is long.
There is another tire which has its bead core enlarged by increasing the number of bead wires relatively while holding the shape of the bead core having a generally hexagonal section. Since this tire has a large extension in its bead core toward the bead toe, it is not expectable to improve the strength of the bead core at the side of the bead toe. Since, moreover, the weight of the bead core is remarkably increased, the effect of preventing the deformation of the bead toe is slight.